Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for generating a driving signal for vibroseis marine sources.
Discussion of the Background
Reflection seismology is a method of geophysical exploration to determine the properties of a portion of a subsurface layer in the earth, which is information especially helpful in the oil and gas industry. Marine reflection seismology is based on the use of a controlled source that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
For marine applications, sources are essentially impulsive (e.g., compressed air is suddenly allowed to expand). One of the most used sources is airguns. An airgun produces a high amount of acoustics energy over a short time. Such a source is towed by a vessel either at the water surface or at a certain depth. The acoustic waves from the airgun propagate in all directions. A typical frequency range of the emitted acoustic waves is between 6 and 300 Hz. However, the frequency content of impulsive sources is not fully controllable, and different sources are selected depending on the needs of a particular survey. In addition, the use of impulsive sources can pose certain safety and environmental concerns.
Thus, another class of sources that may be used is vibratory sources. Vibratory sources, including hydraulically powered sources and sources employing piezoelectric or magnetostrictive material, have been used in marine operations. However, there is no large-scale use of such sources because they have limited power and are not reliable due to the number of moving parts required to generate the seismic waves. A few examples of such sources are now discussed.
A marine vibrator generates a long tone with a varying frequency, i.e., a frequency sweep. This signal is applied to a moving part, e.g., a piston, which generates a corresponding seismic wave. Instantaneous pressure resulting from the movement of plural pistons corresponding to plural marine vibrators may be lower than that of an airgun array, but total acoustic energy transmitted by the marine vibrator may be similar to the energy of the airgun array due to the extended duration of the signal. However, such sources need a frequency sweep to achieve the required energy. Designing such a frequency sweep is now discussed.
U.S. Patent Application Publication No. 20100118647A1, entitled, “Method for optimizing energy output from a seismic vibrator array,” the entire disclosure of which is incorporated herein by reference, discloses two flextensional vibrators (low frequency and high frequency) activated by electro-mechanical actuators and emitting seismic energy at two different depths during a frequency sweep. The vibrators are driven by swept frequency signals, each having a different selected frequency response. Signals such as Maximum Length Sequence (MLS) or Gold Sequence (GS) are also used to drive the vibrators. However, the driving signal of this document does not take into account various physical constraints of the seismic vibrator or the medium in which the vibrator operates.
A non-linear frequency sweep is described in U.S. Pat. No. 6,942,059B2, entitled, “Composite bandwidth marine vibroseis array,” the entire content of which is incorporated herein by reference. This document discloses a method for seismic marine survey using vibrator sources, each of them placed at different depths. The vibrator sources show a level of seismic energy comparable to an airgun array (single depth) by dividing the seismic bandwidth over a plurality of different bandwidths. Each bandwidth is generated by a vibrator array using a non-linear sweep in order to maximize the output energy. However, this document does not consider the various physical constraints of the marine vibroseis array when determining the frequency sweep.
A sweep design method for a seismic land vibrator is also disclosed in U.S. Pat. No. 7,327,633, entitled, “Systems and methods for enhancing low-frequency content in vibroseis acquisition,” the entire content of which is incorporated herein. The patent discloses a method for optimizing sweep signal strength by taking into account a single physical property of a seismic land vibrator, i.e., a stroke limit of the seismic vibrator device. A non-linear sweep is obtained in order to build up the sweep spectral density to achieve a targeted spectrum in the low frequency range. However, other physical properties of the seismic land vibrator, which limit the operation of the land vibrator, are not considered. Further, this patent is directed to a land vibrator, which is different from a marine vibrator.
A more sophisticated sweep design method is disclosed in U.S. patent application Ser. No. 12/576,804, entitled, “System and method for determining a frequency sweep for seismic analysis,” the entire content of which is incorporated herein by reference. This method takes into account not only the plate stroke limit but also other constraints of the land vibrator, e.g., the pump flow limit and the servo valve flow limit. However, this method addresses a land vibrator, which has different characteristics than a marine vibrator, and the method also does not take into consideration specific features of the water environment.
Thus, there is a need to provide a method for designing a driving signal that takes into account constraints of the marine vibrator and, optionally, constraints imposed by the water environment.